The negative regulation of vascular patterning is one of the least understood processes in vascular biology. In higher vertebrates, blood vessels develop throughout most of the embryo. But in one region at the midline surrounding the notochord, despite the presence of multiple positive signals for vessel formation, blood vessels do not form. This midline avascular zone is maintained until the bilateral dorsal aortae fuse to form a single dorsal aorta. We recently found that notochord-derived BMP antagonists serve as negative regulators of vascular growth and are responsible for midline avascular zone genesis. This demonstrates a novel inhibitory role of BMP antagonists in vessel development. However, it does not explain how BMP antagonists inhibit vessel formation in the presence of potent vessel promoting signals, such as VEGF. It also remains uncertain how the midline region changes to permit the paired dorsal aortae to fuse later in development. Our preliminary data show that: a) both notochord and BMP antagonists can suppress the expression of endothelial cell differentiation markers in cultured mesoderm; b) BMP antagonist-treatment of differentiated endothelial cells results in a rapid loss of VEGF receptor Vegfr2 protein (also known as Flk1), while Vegfr2 mRNA levels remain unaffected; and c) the expression of BMP antagonists in the notochord declines as the paired dorsal aortae begin to fuse along the midline. These results lead to the hypotheses that: 1) local suppression of endothelial cell differentiation is a mechanism of avascular zone formation by notochord and BMP antagonists at the midline; 2) BMP antagonist- induced inhibition of differentiated endothelial cells is mediated by post-transcriptional suppression of Vegfr2 protein accumulation; and 3) a developmentally regulated decline of BMP antagonists in the notochord allows the bilateral dorsal aortae to fuse along the midline later in development. We will test these three hypotheses by removing and adding BMP antagonists to naive mesoderm, differentiated endothelial cells, and paired dorsal aortae, and analyzing alteration in resulting endothelial cell differentiation and Vegfr2 levels in endothelial cells as well as dorsal aortae fusion. The studies proposed here will identify a novel signaling mechanism that regulates vessel differentiation and patterning, and build a foundation for rational therapeutics of vascular disorder in adults. PUBLIC HEALTH RELEVANCE: Future therapeutic approaches to regeneration or repair of blood vessels after injury or congenital disease would benefit from a clear understanding of the mechanisms that regulate vessel development. Many positive signaling mechanisms have been identified, but little is known about negative regulation. This project aims to determine, for the first time, a novel inhibitory mechanism that regulates vascular cell differentiation and function.